Too much pressure?

Well my rebuilt extruder didn't last long enough to even make a raft!

I calibrated the Z origin yesterday but when I started extruding today the nozzle ploughed into the polypropylene bed. Thinking I had made some mistake I calibrated it again and it did the same thing. The PTFE barrel can no longer slip in the clamp because it is pinned. The heater barrel can not slip out of its thread because I have a metal ring around the PTFE to stop it swelling. What seems to have happened is that the PTFE barrel has elongated.



It has also bent somewhat. The last PTFE barrel did not elongate significantly but it did swell. I can only think that because I have reduced the nozzle aperture from 0.5mm to 0.3mm the pressure has increased. I didn't notice much change in the motor current though.

I am not sure what to do now. I could make a thicker PTFE barrel but I will have to make a new clamp, which means converting my machine back to a milling machine, or I could drill the hole out to 0.5mm.

A 0.5mm hole gave me ~1.2mm filament which I stretched to 0.5mm. The 0.3mm hole gave me 0.77mm filament at the same extrusion volume rate. It still needed stretching to meet my 0.5mm target. Also it has the disadvantage that the maximum size I can extrude without stretching is now 0.77mm rather than 1.2mm. I didn't get chance to find out what effect less stretching had if any.

The sleeving I used is already looking sad so I ordered some PTFE sleeving to replace it.

Coming back together at last

One of the things that irritated me about the extruder was the time it took to open the pump and close it again. To make it slightly less fiddly I replaced the lock nuts with a pair of threaded steel plates :-



I used Forrest's recipe for cooling fluid while drilling these. It worked a treat, I had no idea you needed cooling when drilling steel to prevent the drill from being burnt.

The springs I used first time around that came from an old CD player weren't quite strong enough but I found some better ones in the hinges of an A3 scanner I dismantled recently. Six of these held the lid open.



Here they are installed on the machine :-



I should be extruding again tomorrow!

Thermal thoughts

I attached the wires of my new heater with some small crimps that I cut from connector pins like this:-



to just leave the crimp part :-



I also soldered them with lead free solder which melts at about 220°C rather than the tin lead solder that I used last time, which melts at 183°C. It could do with being higher still so I might see if I can buy some high temperature solder (301°C) for the next one I make.



Rather than insulate with the recommended heat shrink sleeving, which is only rated for 125°C, I used some high temperature woven sleeving. Even that seems to discolour at 240°C. PTFE sleeving might be better.



I pushed the wires from the thermistor into a two pin connector so that I can easily remove the nozzle.

I had to make a new mounting bracket because when I added the pipe around the PTFE insulator to stop it expanding it made it too big to go through the hole in my previous bracket, which also doubles as my spindle mount when milling.

The bit of aluminium box section I cut it from has been waiting for a new life since it appeared in a prototype electronic photo booth on Tomorrow's World in the early nineties.

I calibrated the thermistor by putting a thermocouple into a second hole in the nozzle with some silicon grease. I also put a second thermocouple inside the barrel. I then put a range of voltages across the heater and waited for it to reach thermal equilibrium before reading the temperatures and the resistance of the thermistor.



Notice how dark the J-B Weld has gone when briefly heated to 240°C.

I made the new heater resistance 7.1Ω compared to 8.6Ω last time because I guessed the acorn nut nozzle would have a larger surface area and hence greater heat loss. In fact it has a remarkably similar temperature power curve to my last nozzle.


Note that the difference between the internal and external temperatures is as much as 25°C. The PTFE tape I used to seal the nozzle may not be helping here. I also measured the outside temperature of the J-B Weld using an IR thermometer which read 242°C when the inside was 245°C and the nozzle was 220°C.

After calibrating the thermistor it seems to track the thermocouple within about 5°C.